The present invention relates to magnet construction. More particularly it relates to a method for the construction of annular permanent magnets, especially but not solely for use in MRI systems, and an apparatus for constructing such magnets.
Magnetic resonance imaging systems are known in the art. The remarkable soft tissue contrast resolution associated with these techniques is invaluable and renders these techniques high appreciation among the medical community.
Basically MRI techniques exploit nuclear magnetism induced on the patient""s tissues, eliciting Radio-Frequency induced signal response which is picked up, analyzed and processed to obtain an image of the imaged region of the patient""s tissue (a very clear explanation of the MRI principles is provided by Joseph P Homak, of the Rochester Institute of Technology, on the World Wide Web, http:/www.cis.rit.edu/htbooks/mri/mri-main.htm, and see also U.S. Pat. No. 5,304,933 (Vavrek et al.), titled SURGICAL LOCAL GRADIENT COIL).
The first stage of MRI involves the aligning of the patient""s tissue nucleons magnetic spins. This is achieved by placing the patient (or the patient""s organ to be imaged) in a strong magnetic field generated by a strong permanent magnet or a super-conductive magnet. It is imperative that the magnetic field in the field of view of the system be homogeneous, as distorted field may result in the distortion of the image and the appearance of artifacts in the image.
Super-conductive magnets can produce extremely strong and substantially homogeneous magnetic fields (typical magnet strength in known MRI systems may be as high as 2 Tesla). See for example U.S. Pat. No. 4,924,186 (Matsutani) titled MAGNET APPARATUS FOR USE IN MAGNETIC RESONANCE IMAGING SYSTEM. These are large superconductor magnets, which take up a large space (sometimes as large as a room), are expensive and require high operating and maintenance costs. The large size of these magnets prevents any access to the patient.
However, recently it was realized that whole body imaging is not necessary for the performance of an interventional medical procedure on a patient in an MRI system. It has been realized that, in fact, a machine with a restricted field of view performs satisfactorily in such a setting and can be built in a more efficient and economical fashion than one built for accommodating a whole body. Furthermore, in order to leave an open access to reach conveniently the part of the body on which the intervention is performed, compact magnet assemblies were introduced.
Israel Pat. Appl. No. 119558 (Katznelson et al.) filed Nov. 4, 1996, discloses a compact, mobile, intra-operative MRI System, which includes a host computer coupled to a central electronics system which may be coupled to different MRI probes.
In U.S. Pat. No. 5,428,292 (Dorri et al.), filed Apr. 29, 1994, A pancake-like MRI magnet was disclosed, presenting a relatively narrow lateral cross-section.
Usually permanent magnet assemblies for MRI systems incorporate ferromagnetic structures for the creation of return paths of the magnetic flux. Attaching ferromagnetic (usually iron) annular plates on the surface of the magnet facing the patient act as magnetic field uniformity enhancement.
U.S. Pat. No. 5,900,793 (Katznelson et al.), filed Jul. 23, 1997, titled PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONS, described, inter alia, compact permanent magnet assemblies for use in medical applications, including MRI and/or MRT (Magnetic Resonance Therapy). It consists of a plurality of annular concentric magnets, spaced apart along their axis of symmetry. The magnet assemblies disclosed in that patent are not provided with a ferromagnetic structure.
Permanent magnets are made of non-rare or rare earth magnetic materials. Non-rare earth magnets include Alnico (Aluminum-Nickel-Cobalt) magnets and Ceramic (Strontium and Barium Ferrite) magnets. Rare earth magnets include Smxe2x80x94Co (Samarium-Cobalt) magnets and Ndxe2x80x94Fexe2x80x94B (Neodymium-Iron-Boron) magnets.
The conventional manufacturing method of a permanent magnet involves compressing the magnet material, which is available in the form of powder, shaping it into a predetermined shape, and then magnetizing it by placing it in an extremely strong electromagnetic field (in the order of 3 Tesla). This powerful electromagnetic field, which permanently aligns the magnetic dipoles of the matter is generated by passing powerful electric current produced by discharging a large number of capacitors in a predetermined switching sequence through a copper or super-conductive coil within which the item to be magnetized is placed. See, for example, U.S. Pat. No. 5,250,255 (Segawa et al.), titled METHOD FOR PRODUCING PERMANENT MAGNET AND SINTERED COMPACT AND PRODUCTION APPARATUS FOR MAKING GREEN COMPACTS.
MRI magnets may be cylindrical in their shape, suitable for reception of a patient within the internal space of the cylinder. as illustrated in U.S. Pat. No. 5,659,250 (Domigan et al.). In interventional MRI systems, which require that the patient be accessible to the medical staff, when positioned within the magnetic field, the magnetic field is usually produced by two magnet assemblies that are positioned opposite each other, allowing reception of the patient in between. MRI systems can also utilize a single magnet to produce an image.
Permanent magnets used in MRI systems that require accessibility of the patient are usually annular in their shape. In order to prevent substantial eddy currents induced by the strong magnetic field, MRI magnets are usually segmented, formed from magnetic segments, and glued to each other using a non-conductive adhesive. Thus the generation of eddy currents over the whole magnet is prevented, limiting possible eddy current induction to the segments.
The size of magnets produced in this manner is limited by the size of the coil used for magnetization, which itself is limited by the minimal electromagnetic filed strength required for effective magnetization. It is noted that in order to effectively magnetize the magnet material it has to be subjected to a strong magnetic field that generates magnetic flux within the material exceeding the saturation flux (the flux at which al the magnetic dipoles within the material align). For Ndxe2x80x94Fexe2x80x94B the saturation magnetic flux is about 1.35 Tesla, and for practical reasons it is customary to subject the magnetic material to magnetic fields of twice or more the stated flux.
In view of the above mentioned considerations it is therefore why small magnetic rings are produced in the following manner first, the magnet segments are shaped roughly to form sectors of the ring. Then their sides, designated to be glued to other segments, are ground and the segments are bonded together (using an insulating adhesive) to form the ring. Then the ring undergoes further mechanical processing to bring it to its final shape and dimensions, and finally the whole construction is magnetized.
The manufacturing procedure described above is followed sequentially in the provided order, for if the segments were to be magnetized prior to their joining together, it would be very difficult to adhere the magnetized segments together. As the magnetized segments are drawn closer strong magnetic repulsion forces act on the magnetized segments attempting to flip the segments over, so as to bring opposite polarities to face each other. Therefore enormous forces are required to position the segments adjacent each other and hold them firmly while the adhesive hardens.
The problem with the method discussed above is that it is impossible to control or predict the level of homogeneity of the magnetic field of the constructed magnet that would be attained in this process. In MRI applications it is imperative that the produced magnet possessed a substantially homogeneous magnetic field. However irregularities in the magnetic material, as well as flaws in the magnetization process could render the manufactured magnet non-homogeneous and therefore useless. The larger the magnetized object is the greater the risk of obtaining irregularities in its induced magnetic field.
Due to the limitations imposed on the size of objects that can be magnetized by coils, it is evident that constructing larger annular magnets can only be achieved by joining together previously magnetized segments. Furthermore, it was found that from magnetic field considerations (the major consideration in the construction of MRI magnets) it is far better to select a number of segments from a group of magnetized segments possessing same magnetic strength, than to magnetize a whole annular magnet, and be faced with the possibility of substantial irregularities in the magnetic field.
It is therefore the purpose of the present invention to provide a novel method and apparatus for the manufacture of annular permanent magnets, suitable for MRI applications involving the construction of magnets from previously magnetized segments. Thus greater magnetic field homogeneity is achieved and the risk of manufacturing faulty magnets is reduced, hence reducing undesired disqualification of manufactured magnets for MRI use.
In U.S. Pat. No. 5,659,250 (Domigan et al.), titled FULL BRICK CONSTRUCTION OF MAGNET ASSEMBLY HAVING A CENTRAL BORE, filed in 1997, there was disclosed a permanent magnet assembly having a central eliptical bore, formed of a plurality of eliptically shaped sections. Each section is subdivided into a plurality of segments in which each segment is constructed of bricks of magnetic material. The bricks are arranged parallel to a common plane parallel to the bore axis and magnetized with magnetization vector oriented in a common direction perpendicular to the plane.
Note that the magnetization vectors described by Domigan are all parallel to the magnet annular plane, and it is therefore impossible to magnetize the entire magnet at once. Furthermore, as a consequence of the magnetization direction, the magnet segments do not experience repulsion forces as would magnet segments whose magnetization direction is perpendicular to the annular plane of the magnet experience in a similar arrangement. See also U.S. Pat. No. 5,148,138 (Miyata), titled CYLINDRICAL MAGNET APPARATUS SUITABLE FOR NMR IMAGING, which describes magnetic arrangement involving magnet segments whose magnetization vectors are parallel to the annular plane of the magnet.
It is clear that the problem of repulsion forces acting between magnet segments magnetized perpendicular to the annular plane of the magnet, as they are brought doser, is substantial and needs to be overcome, and the present invention discloses the ways of achieving this.
It is an object of the present invention to provide such a method and apparatus that would facilitate safe positioning of magnetized segments in a prearranged positions so as to form an annular magnet, and allow safe handling of the segments and binding them together to form the annular magnet.
There is therefore thus provided, in accordance with a preferred embodiment of the present invention an apparatus for the construction of a segmented annular permanent magnet from a predetermined number of previously magnetized segments of predetermined shape and size, said apparatus comprising:
a rotatable carrousel comprising an upper platform and a lower platform, said platforms placed substantially parallel, adapted to receive between them, in an annular arrangement, said previously magnetized segments,
at least two of a plurality of removable ferromagnetic segments adapted to be mounted and dismounted from said carrousel platforms, designated to shorten the magnetic flux of the previously magnetized segments when mounted on the carrousel;
an introducing means, adapted to introduce said previously magnetized segments onto said carrousel; and
a removing means, adapted to facilitate the removal of said removable ferromagnetic segments from the carrousel.
Furthermore, in accordance with a preferred embodiment of the present invention, said platforms are detachable.
Furthermore, in accordance with a preferred embodiment of the present invention, said carrousel is further provided with fastening means for holding each of said previously magnetized segments in position once they are positioned.
Furthermore, in accordance with a preferred embodiment of the present invention, said apparatus further comprises a supporting body on which said rotatable carrousel, introducing means and removing means are mounted.
Furthermore, in accordance with a preferred embodiment of the present invention, said apparatus is designed to withstand a load of up to 600 kg.
Furthermore, in accordance with a preferred embodiment of the present invention, said apparatus, except said ferromagnetic segments, is made from diamagnetic material.
Furthermore, in accordance with a preferred embodiment of the present invention, said material is selected from annealed austonitic stainless steel, bronze, aluminum and glass-fiber.
Furthermore, in accordance with a preferred embodiment of the present invention, said supporting body is provided with a frame on its bottom, said frame provided with bores for bolts in order to secure the supporting body to a floor.
Furthermore, in accordance with a preferred embodiment of the present invention, said supporting body is provided with two lateral bores as to allow lifting and repositioning of said apparatus by means of inserting bars through said bores and lifting them.
Furthermore, in accordance with a preferred embodiment of the present invention, said supporting body is provided with vertical grooves, each on either side, so as to allow the installation of shielding to protect an operator of said apparatus from inadvertent jettisoning of magnetized segments.
Furthermore, in accordance with a preferred embodiment of the present invention, said apparatus is further provided with a drum adapted to be mounted on said carrousel and provide radial support to said previously magnetized segments.
Furthermore, in accordance with a preferred embodiment of the present invention, said drum comprises a plurality of segments, each defining a section of the drum, and coaxially mounted on said carrousel, between said platforms.
Furthermore, in accordance with a preferred embodiment of the present invention, said drum segments are spaced apart with gaps between them to allow convenient release of the drum segments thus facilitating convenient dismounting of said segmented annular permanent magnet after its construction is completed.
Furthermore, in accordance with a preferred embodiment of the present invention, said drum is provided with an upper annular recess and a lower annular recess so as to present shoulders for the accommodation of an internal rim of said parallel platforms.
Furthermore, in accordance with a preferred embodiment of the present invention, said drum is fastened to said parallel platforms by means of bolts.
Furthermore, in accordance with a preferred embodiment of the present invention, said at least two of a plurality of removable ferromagnetic segments comprise a plurality of annular segments.
Furthermore, in accordance with a preferred embodiment of the present invention, said at least two of a plurality of removable ferromagnetic segments are coupled to said parallel platforms by means of bolts.
Furthermore, in accordance with a preferred embodiment of the present invention, said carrousel is provided with locking means adapted to look said carrousel in position at a number of predetermined equiangular positions, said number equal to said number of previously magnetized segments.
Furthermore, in accordance with a preferred embodiment of the present invention, said locking means comprise a locking pin, adapted to be inserted into each of a number of matching bores provided on one of said platforms.
Furthermore, in accordance with a preferred embodiment of the present invention, said number of predetermined equiangular positions is in the range between 12 to 18.
Furthermore, in accordance with a preferred embodiment of the present invention, said number of predetermined equiangular positions is 16.
Furthermore, in accordance with a preferred embodiment of the present invention, said introducing means, removing means and carrousel are aligned along a substantially straight line.
Furthermore, in accordance with a preferred embodiment of the present invention, said introducing means comprises an arm coupled to a support block provided with clamping arms for holding one of said previously magnetized segments.
Furthermore, in accordance with a preferred embodiment of the present invention, said arm is a hydraulic arm.
Furthermore, in accordance with a preferred embodiment of the present invention, said clamping arms comprise two vertically oriented clamping arms.
Furthermore, in accordance with a preferred embodiment of the present invention, the apparatus further comprises an introduction chamber providing a predetermined pathway through which the magnetized segment is introduced to said carrousel.
Furthermore, in accordance with a preferred embodiment of the present invention, said introduction chamber is further provided with parallel grooves adapted to slidingly receive said clamping arms, and thus guiding said block along said predetermined pathway.
Furthermore, in accordance with a preferred embodiment of the present invention, said platforms are each provided with equiangular radial grooves, equal in number to said predetermined number of previously magnetized segments, and adapted to slidingly receive said clamping arms, thus guiding said segment to a predetermined position on said carrousel.
Furthermore, in accordance with a preferred embodiment of the present invention, there is provided, in accordance with a preferred embodiment of the present invention, a method for the construction of a segmented annular permanent magnet from a predetermined number of previously magnetized segments said method comprising:
a. selecting a predetermined number of previously magnetized annular segments;
b. arranging said previously magnetized annular segments in an annular arrangement;
c. coupling said previously magnetized annular segments together to form an annular permanent magnet.
Furthermore, in accordance with a preferred embodiment of the present invention, the step of selecting a predetermined number of previously magnetized annular segments further including matching said segments magnetic strength to ensure selection of segments of substantial magnetic strength homogeneity, having no or little variance in the magnetic field strength.
Furthermore, in accordance with a preferred embodiment of the present invention, the magnetized segments magnetic field strength variance is not greater than 50 ppm.
Furthermore, in accordance with a preferred embodiment of the present invention, said previously magnetized segments are introduced one at a time.
Furthermore, in accordance with a preferred embodiment of the present invention, said method further comprises a step of providing a carrousel onto which said previously magnetized segments are mounted and affixed in said annular arrangement
Finally. in accordance with a preferred embodiment of the present invention, said previously magnetized segments are each affixed in position on said carrousel.